The present invention relates to a ripple and noise filter circuit, and more particularly to a ripple and noise filter circuit with a wide frequency operating range for use with very high voltage DC power supplies.
The type of filter circuit (or simply filter) to which the present invention relates is commonly used for filtering unwanted ripple and noise voltages from the DC output of a DC power supply incorporating an AC to DC converter circuit. A description of a switching mode converter circuit of the type with which the filter circuit of the present invention may be used is disclosed in U.S. Pat. No. 4,475,149.
Ripple voltages are periodic small voltage modulations of the DC output voltage of the converter or DC power supply. A ripple voltage appears, on an oscilloscope, as a small wave-shaped voltage which "rides" on top of the output DC voltage. The source and cause of ripple is most often the AC power input which includes AC voltage components which unavoidably are coupled onto the DC output terminals of the converter. Ripple voltages are, therefore, primarly low frequency waveforms, in the range of from 60 cycles to several hundred cycles per second.
Voltage or noise spikes, on the other hand, are voltage bursts which momentarily shoot above or below the steady state DC voltage level of the converter. The voltage bursts are very narrow and last for a very short time, usually in the order of nanoseconds or, at most, several microseconds. The spikes tend to appear periodically or spuriously on the DC output line at intervals which are determined by, among other factors, the converter's chopping frequency and/or the load's characteristics. Voltage spikes are characterized in that they possess very sharply rising and falling edges. Consequently, the frequency spectrum of the noise spikes extends well into the megahertz frequency range and may include voltage noise components with a frequency of up to 20 to 50 megahertz.
To remove, or more correctly to reduce, the ripple and noise from the DC output, a filter circuit is interposed between the DC output terminals of the converter and the load. A load refers generally to an electrical device or component which is powered by, uses, or operates off the DC voltage. The load may be resistive or reactive. It may be a device such as a simple DC light bulb or a complex electronic circuit or motor in which current requirements fluctuate wildly according to a complex set of criteria.
In any event, the filter's function is to monitor the output voltage, detect unwanted voltage variations due to ripple or spikes and compensate for the ripple and spikes by "modifying" the output voltage in a manner which negates the effect of ripple/noise to produce a "clean" DC output.
To do so, the filter must have an operating range over a wide frequency band which is as wide as the band-width of the ripple or noise voltages. In the applications to which the present invention is particularly relevant, the frequency range or bandwidth is from 50 hertz to several megahertz. The filter must also be capable of operating in the environment of the high voltage converter in which voltage levels of hundreds, and perhaps several thousand volts, are present. Additionally, the filter circuit must not add significant weight, space or costs to the overall converter or DC supply.
The prior art teaches two general types of filter circuits for filtering ripple and noise voltages from power supplies. The structure, operation and limitations of these filters, particularly in relation to their use with a high voltage converter, will be discussed below. The two filter types are known as the passive filter and the active filter designs.
In a passive filter, passive components (resistors, capacitors, and inductors) are used exclusively. The filter design offers the advantage of relatively inexpensive discreet components which can be readily selected and specified to withstand high voltages.
Nevertheless, the use of a passive filter in conjunction with high voltage DC voltages is impractical for the following reasons. A passive filter usually includes a filtering inductor and a filtering capacitor. The inductor and the capacitor must be selected to have a resonating frequency which is significantly below the frequency range of operation of the filter. This is important because if ripple and noise voltages include frequency components that fall in the range of resonance of the filter, these noise and ripple voltages will be accentuated instead of being eliminated. To ensure that the filter resonates at very low frequencies, the filters characteristic impedance, which is related to the ratio of the inductor divided by the capacitor volume, must be kept very low. This is difficult to achieve in high voltage power supplies, particularly if the volume is limited.
Furthermore, the filter must be well damped so that changes in output current or input line voltage at the filter's resonant frequency are not accentuated. Therefore, a damping resistor with a value equal to or less than the low characteristic impedance is included in the filter. To avoid excessive dissipation in the damping resistor, a blocking capacitor must be also added in series with the damping resistor.
The component values which would satisfy both criteria, result in a filter circuit which is usually too large to fit within the volume allocated to high performance power supplies. In addition, it is more difficult to design a filter with a flat frequency response over a wide frequency range by using only passive elements.
Filter designers prefer, therefore, to use active elements, including amplifiers to provide an active ripple and noise filter. With an active filter, the frequency response or the performance of the filter over a desired frequency range can be more accurately controlled. However, active filters include delicate electronic components which cannot withstand high voltages. In this connection, it should be made clear that certain active elements are capable of operation with voltages of perhaps up to 20 or 30 volts and their use even in lower voltage supplies involves design compromises.
In one active filter prior art embodiment, the filter includes a sensing circuit for generating the sense signal for an amplifier. The amplifer produces an output signal which is coupled back onto the DC output by means of an isolation transformer.
The isolation transformer which receives the output from the amplifier at its primary winding must be able to reproduce the signal on its secondary winding without distortion. This is achievable only with a transformer which has a frequency operating range as wide as the bandwidth of the amplifier itself. A transformer with a frequency range of from 100 hertz to over 1 megahertz must meet the following requirements:
(1) it must sustain relatively high DC current in its secondary; PA1 (2) it must have a relatively large primary inductance to provide low frequency response without overloading the active filter's amplifier output; PA1 (3) it must include sufficient insulation to permit it to reliably withstand high secondary DC voltages; and PA1 (4) it must exhibit very low primary to secondary leakage inductance, particulary since the secondary winding is "loaded" by the converter's output capacitance.
The above requirements mandate a transformer of a size and cost which makes its use in high voltage power supplies undesirable.
In view of the foregoing discussion, it is seen that the prior art has not been successful in providing a suitable ripple and noise filtering circuit without incurring severe space, weight, and cost penalties.